1. Field of the Invention
The invention pertains to an edge-covering material and a process for preparing a flexographic printing form, whereby a photopolymerizable printing plate bearing a laser mask coating is premounted on a sleeve or cylinder, a laser mask is prepared, the photopolymerizable printing plate is exposed through the laser mask, and the exposed printing plate is washed off and dried.
2. Description of Related Art
Photopolymerizable printing plates are known to be used for preparing flexographic printing forms. The printing surface is produced by exposing imagewise, through a photographic original, a layer photopolymerizable by actinic radiation, and subsequently removing the unexposed, unphotopolymerized areas of the printing plate. Examples are found in the following patents: DE-C 22 15 090, EP-A 0 322 585 and U.S. Pat. No. 4,323,636.
Imagewise exposure of the photopolymerizable printing plates requires a photographic original, which is a mask having transparent and opaque areas in the form of an image to be reproduced. This photographic original is usually a photographic negative of the image to be reproduced. If corrections are required for the final image, a new original must be prepared. The same original can yield varying results, depending on processing conditions (temperature, humidity), so that registration problems can arise in the printing form montage. To simplify this time-consuming and costly process, photopolymerizable printing plates are provided with an additional layer that can be processed into an integrated original. Such materials having infrared (IR)-ablative, ultraviolet (UV)-absorbing layers and a process for their preparation and processing are described in WO 94/03838, WO 94/03839 and WO 96/16356. The integrated photomask is prepared by laser exposure.
The development of such materials is directed to distinctly simplifying the preparation of accurately registered cylinder shells, the so-called sleeves, and cylinders, in addition to direct processing of digital information and higher resolution in the resulting flexographic printing forms. This results from the montage of sections of laser-writeable, photopolymerizable, flexographic printing plates on the sleeve or cylinder. The sections can be positioned with a high degree of tolerance, because exposure with the precisely controlled laser head yields accurately registered alignment of the elements of the ultimate printed image. Sleeves or cylinders thus recorded are then exposed with UV in a circular exposure unit and washed off. Unfortunately, polymerized ridges remain on the edges of the plate sections and subsequently print in an undesirable manner.
During the preparation of ready-to-print cylinders or sleeves from sections of conventionally finished flexographic printing plates, it is possible to remove the unwanted ridges that result from exposure of the printing plate edges. Such plates can be cut before the montage is assembled. Even with laser-processed printing plates, there is the possibility of subsequent removal of undesirable ridges, for example, by cutting with sharp knives. However, this operation takes time, the danger of distorting the printing forms is very great, especially if the subject runs close to the edge of the printing form, and removing the severed ridge without leaving a residue is very expensive.
Therefore, the problem involved in the present invention is to avoid the disadvantages of the current state of the art and to reliably, rapidly and simply prevent the ridges from occurring in processing laser-processible, flexographic printing plates in a circular exposure unit.